Audio enabled in Preview Panel?
Does audio play when you press the . on the numeric keypad or pick Composition>Preview>Audio?
Are you using the 0 key on the numeric keypad or the Preview button on the Preview Panel to generate a ram preview?
Will a standard wav file play?
Are you an AE Newbie?
Did you read this FAQ on audio playback?
Did you read the AE Help file on Audio Preview and Playback?